The motion picture studies on the differentiation and maturation of mouse granulocytes was completed. A promyelocyte required six successive cell divisions in a period of 5 days to produce a total of 64 granulocytes. A "weaning" stage of 1-2 days was observed after the last cell division. Another 3-4 days was required for the cells to become the mature granulocytes. A new technique for the quantitative estimation of the growth of granulocytes and macrophages has been developed. Endotoxin-treated (in vivo) mouse serum and endotoxin-treated (in vivo) mouse lung tissue conditioned medium showed strong stimulating effect on the growth of granulocytes in cultures. Cytochemical studies on the so-called "novel cell" revealed that these cells were probably a type or a stage of macrophages. Growth of Mycobacterium lepraemurium in cultures of macrophages obtained from various tissue sources, i.e., bone marrow, spleen, and blood leukocytes, was accomplished. The effect of two anti-amebic drugs on murine leprosy was studied in mice. Both drugs, broxyquinoline and broxaldine, showed little activity.